1. Field of the Invention
The present invention relates to an improvement in a field effect transistor, and particularly to a field effect transistor which provides linearity to its attenuation characteristics and also improves its distortion factor to transmit a signal in a good condition when it is employed as an attenuator, by way of example.
2. Description of the Prior Art
When, for example, an insulated gate type (MOS-type) field effect transistor (hereinafter referred to as an FET) is used as a variable impedance element and also used in an attenuator circuit, a circuit arrangement such as depicted in FIG. 1 is generally adopted. In FIG. 1, reference numeral 1 designates a resistor interposed between input and output terminals 2 and 3 of a signal transmitting path l and a signal S.sub.1 supplied to the input terminal 2 is controlled of the attenuation condition of its transmitting amount by an FET 10 connected in parallel to the transmitting path l. In other words, the impedance between source and drain of the FET 10 is controlled by a control voltage (gate voltage) V.sub.G applied to its gate electrode 4 thereby to control the attenuation amount of the signal S.sub.1.
In this connection, as is well known, the attenuation amount of an output signal S.sub.2 controlled by the FET 10, that is, its gain g.sub.1 is expressed as follows: ##EQU1## where R is the resistance value of the resistor 1 and .beta. is the proportional constant to determine the channel conductance of the FET 10 which is obtained by the following equation: ##EQU2## where .epsilon..sub.OX is the dielectric constant of a gate oxide film, T.sub.OX is the thickness of the gate oxide film, .mu. is the mobility of a carrier, L is the length of a channel, and W is the width of a channel.
Accordingly, the equation (1) is expressed by curves such as in FIG. 2 wherein the abscissa represents the gate voltage V.sub.G and the ordinate represents the gain or attenuation amount g. A curve 11 indicates the attenuation curve in the case of .beta. = 1.0 mg/V and R = 3.4 K.OMEGA. and a curve 12 represents the attenuation curve in the case when only the value of R is changed to 680 K.OMEGA..
As will be apparent from FIG. 2, a curve 12, by way of example, is characteristic of large attenuation amount, but if the control voltage V.sub.G is small, that is, the control voltage V.sub.G decreases to the vicinity of the threshold voltage V.sub.th of the FET 10, the attenuation amount is changed so quickly to increase the distortion factor. On the other hand, in the case of the curve 11, since the attenuation amount is not changed so quickly, the distortion factor is not influenced so much. However, there are drawbacks that its linearity is deteriorated and also the attenuation amount of the whole device can not be increased.
In any case, the attenuation circuit such as shown in FIG. 1 will not satisfy all the characteristics of linearity, attenuation amount and distortion factor.